Abstract. Marine biogenic particle contributions to atmospheric aerosol concentrations are not well understood though they are important for determining cloud optical and cloud nucleating properties. Here we examine the relationship between marine aerosol measurements with satellite and model fields of ocean biology and meteorological variables during the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES). NAAMES consisted of four field campaigns between November 2015 and April 2018 that aligned with the four major phases of the annual phytoplankton bloom cycle. The FLEXPART Lagrangian particle dispersion model is used to connect these variables spatiotemporally to ship-based aerosol and dimethyl sulphide (DMS) observations. We find that correlations between some aerosol measurements with satellite measured and modelled variables increase with increasing trajectory length, indicating biological and meteorological processes over the air mass history are influential to measured particle properties and that using only spatially contemporaneous data would miss correlative connections that are lagged in time. In particular, the marine non-refractory organic aerosol mass correlates with modelled marine net primary production when weighted by 5-day air mass trajectory residence time (r = 0.62). This result indicates non-refractory organic aerosol mass is influenced by biogenic volatile organic compound (VOC) emissions from photosynthesis by phytoplankton stocks during advection into the region. This is further supported by the correlation of non-refractory organic mass with 2-day residence-time-weighted chlorophyll-a (r = 0.39), a proxy for phytoplankton abundance, and 5-day residence-time-weighted downward shortwave forcing (r = 0.58), a requirement for photosynthesis. In contrast, DMS (formed through biological processes in the seawater) and primary marine aerosol (PMA) concentrations showed better correlations to explanatory biological and meteorological variables weighted with shorter air mass residence times, which reflects their localized origin as primary emissions. Aerosol submicron number and mass negatively correlate with sea surface wind speed. The negative correlation is attributed to enhanced PMA concentrations under higher wind speed conditions. We hypothesized that the elevated total particle surface area associated with high PMA concentrations leads to enhanced rates of VOC condensation onto PMA. Given the high deposition velocity of PMA, relative to submicron aerosol, PMA can limit the accumulation of secondary aerosol mass. This study provides observational evidence for connections between marine aerosols and underlying ocean biology through complex secondary formation processes, emphasizing the need to consider airmass history in future analyses.
Abstract. North American pollution outflow is ubiquitous over the western North Atlantic Ocean, especially in winter, making this location an ideal natural laboratory for investigating the impact of precipitation on aerosol particles along air mass trajectories. We take advantage of observational data collected at Bermuda to seasonally assess the sensitivity of aerosol mass concentrations and volume size distributions to accumulated precipitation along trajectories (APT). The mass concentration of particulate matter with aerodynamic diameter less than 2.5 µm normalized by the enhancement of carbon monoxide above background (PM2.5/∆CO) at Bermuda was used to estimate the degree of aerosol loss during transport to Bermuda. Results for December–February (DJF) show most trajectories come from North America and have the highest APTs, resulting in significant reduction (by 53 %) in PM2.5/∆CO under high APT conditions (> 13.5 mm) relative to low APT conditions (< 0.9 mm). Moreover, PM2.5/∆CO was most sensitive to increases in APT up to 5 mm (−0.044 µg m−3 ppbv−1 mm−1) and less sensitive to increases in APT over 5 mm. While anthropogenic PM2.5 constituents (e.g., black carbon, sulfate, organic carbon) decrease with high APT, sea salt in contrast was comparable between high and low APT conditions owing to enhanced local wind and salt emissions in high APT conditions. The greater sensitivity of the fine mode volume concentrations (versus coarse mode) to wet scavenging is evident from AERONET volume size distribution data. A combination of GEOS-Chem model simulations of 210Pb submicron aerosol tracer and its gaseous precursor 222Rn reveal that (i) surface aerosol particles at Bermuda are most impacted by wet scavenging in winter/spring (due to large-scale precipitation) with a maximum in March, whereas convective scavenging plays a substantial role in summer; and (ii) North American 222Rn tracer emissions contribute most to surface 210Pb concentrations at Bermuda in winter (~75–80 %), indicating that air masses arriving at Bermuda experience large-scale precipitation scavenging while traveling from North America. A case study flight from the ACTIVATE field campaign on 22 February 2020 reveals a significant reduction in aerosol number and volume concentrations during air mass transport off the U.S. East Coast associated with increased cloud fraction and precipitation. These results highlight the sensitivity of remote marine boundary layer aerosol characteristics to precipitation along trajectories, especially when the air mass source is continental outflow from polluted regions like the U.S. East Coast.
Satellite observations have revealed an enhanced aerosol layer near the tropopause over Asia during the summer monsoon, called the Asian Tropopause Aerosol Layer (ATAL). In this work, aerosol particles in the ATAL were collected with a balloon-borne impactor near the tropopause region over India, using extended duration balloon flights, in summer 2017 and winter 2018. Their chemical composition was further investigated by quantitative analysis using offline ion chromatography. Nitrate (NO3−) and nitrite (NO2−) were found to be the dominant ions in the collected aerosols with values ranging between 87–343 ng/m3 STP during the summer campaign. In contrast, sulfate (SO42−) levels were found above the detection limit (> 10 ng/m3 STP) only in winter. In addition, we determined the origin of the air masses sampled during the flights through analysis of back trajectories along with convective influence. The results obtained therein were put into a context of large-scale transport and aerosol distribution with GEOS-Chem chemical transport model simulations. The first flight of summer 2017 which sampled air mass within the Asian monsoon anticyclone (AMA), influenced by convection over Western China, was associated with particle size radius (0.05–2 μm). In contrast, the second flight sampled air mass at the edge of the AMA associated with larger particle size radius (> 2 μm) with higher nitrite concentration. The sampled air masses in winter 2018 were likely affected by smoke from the Pacific Northwest fire event in Canada, which occurred 7 months prior to our campaign, leading to concentration enhancements of SO42− and Ca2+. Overall, our results suggest that nitrogen-containing particles represent a large fraction of aerosols populating the ATAL, in agreement with the results from aircraft measurements during the StratoClim campaign. Furthermore, GEOS-Chem model simulations suggest that lightning NOx emissions had a significant impact on the production of nitrate aerosols sampled during the summer 2017.
Abstract. Satellite observations have revealed an enhanced aerosol layer near the tropopause over Asia during the summer monsoon, called the Asian Tropopause Aerosol Layer (ATAL). In this work, aerosol particles in the ATAL were collected with a balloon-borne impactor near the tropopause region over India, using extended duration balloon flights, in summer 2017 and winter 2018. Their chemical composition was further investigated by quantitative analysis using offline ion chromatography. Nitrate (NO3−) and nitrite (NO2−) were found to be the dominant ions in the collected aerosols with values ranging between 87–343 ng/m3 STP during the summer campaign. In contrast, sulfate (SO42−) levels were found above the detection limit (> 10 ng/m3 STP) only in winter. In addition, we determined the origin of the air masses sampled during the flights through analysis of back trajectories along with convective influence. The results obtained therein were put into a context of large-scale transport and aerosol distribution with GEOS-Chem chemical transport model simulations. The first flight of summer 2017 which sampled air mass within the Asian monsoon anticyclone (AMA), influenced by convection over Western China, was associated with particle size radius (0.05–2 μm). In contrast, the second flight sampled air mass at the edge of the AMA associated with larger particle size radius (> 2 μm) with higher nitrite concentration. The sampled air masses in winter 2018 were likely affected by smoke from the Pacific Northwest fire event in Canada, which occurred 7 months prior to our campaign, leading to concentration enhancements of SO42− and Ca2+. Overall, our results suggest that nitrogen-containing particles represent a large fraction of aerosols populating the ATAL, in agreement with the results from aircraft measurements during the StratoClim campaign. Furthermore, GEOS-Chem model simulations suggest that lightning NOx emissions had a significant impact on the production of nitrate aerosols sampled during the summer 2017.
We quantitatively analyzed the space heterogeneity of land use degree in the Jiuxiang River watershed.This is of great significance to land consolidation and sustainable use of land resources in the urbanized watershed.Jiuxiang River watershed in Nanjing City,as a case study area,its two landscape classification maps were conducted from 2003 and 2009 by remote sensing images interpretation,which were used as the main data source for assessment of land use degree.Spatial heterogeneity characteristics of land use degree in the study area were analyzed during 2003-009 by means of spatial statistics like spatial autocorrelation and semi-variance analysis.The results showed that with the increase of spatial distance,the spatial correlation of land use degree showed a downward trend in Jiuxiang River basin.The high value area of land use degree was mainly located in the northwest Xianlin College Town and Shibu Bridge area,and the low value area mainly gathered in the northern Qinglongshan Mountain,the central Linshan Mountain and the northern West Lake Park area.Land use degree was mainly at the level of moderate in Jiuxiang River Basin from 2003 to 2009,accounting for about 40% of the watershed area.Affected by the urbanization process,the spatial heterogeneity of land use degree changed obviously,and the watershed land use degree showed the trend of conversion to stronger and weaker,the distribution area had an increase of 1.13% and 4.27%,respectively in six years.The land use degree in Xianlin College Town area enhanced,showed a trend of rapid expansion,but the land use degree slightly decreased in Jiuxiang River headwater,along the downstream coast and the West Lake Park.So we should establish a watershed ecological planning or land use planning to reduce the land use degree by controlling the density of building lands strictly,protecting forestlands,and enhancing the landscape construction.
Abstract. Errors in chemical transport models (CTMs) interpreting the relation between space-retrieved tropospheric column densities of nitrogen dioxide (NO2) and emissions of nitrogen oxides (NOx) have important consequences on the inverse modeling. They are however difficult to quantify due to lack of adequate in situ measurements, particularly over China and other developing countries. This study proposes an alternate approach for model evaluation over East China, by analyzing the sensitivity of modeled NO2 columns to errors in meteorological and chemical parameters/processes important to the nitrogen abundance. As a demonstration, it evaluates the nested version of GEOS-Chem driven by the GEOS-5 meteorology and the INTEX-B anthropogenic emissions and used with retrievals from the Ozone Monitoring Instrument (OMI) to constrain emissions of NOx. The CTM has been used extensively for such applications. Errors are examined for a comprehensive set of meteorological and chemical parameters using measurements and/or uncertainty analysis based on current knowledge. Results are exploited then for sensitivity simulations perturbing the respective parameters, as the basis of the following post-model linearized and localized first-order modification. It is found that the model meteorology likely contains errors of various magnitudes in cloud optical depth, air temperature, water vapor, boundary layer height and many other parameters. Model errors also exist in gaseous and heterogeneous reactions, aerosol optical properties and emissions of non-nitrogen species affecting the nitrogen chemistry. Modifications accounting for quantified errors in 10 selected parameters increase the NO2 columns in most areas with an average positive impact of 18% in July and 8% in January, the most important factor being modified uptake of the hydroperoxyl radical (HO2) on aerosols. This suggests a possible systematic model bias such that the top-down emissions will be overestimated by the same magnitude if the model is used for emission inversion without corrections. The modifications however cannot eliminate the large model underestimates in cities and other extremely polluted areas (particularly in the north) as compared to satellite retrievals, likely pointing to underestimates of the a priori emission inventory in these places with important implications for understanding of atmospheric chemistry and air quality. Note that these modifications are simplified and should be interpreted with caution for error apportionment.
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